The invention relates to semiconductor devices. More particularly the invention relates to improvements in the configuration of circuits constructed from semiconductor devices to reduce leakage current in the circuits.
Throughout the specification, P and N-channel MOS (metal oxide semiconductor) devices (PMOS and NMOS) are described in terms of their respective gate, drain and source nodes to help clarify the structure and operation of the alternative embodiments. PMOS devices transmit positive current when the signal on the gate is low, and cease transmitting positive current when the signal on the gate is high. NMOS devices transmit positive current when the signal on the gate is high, and cease transmitting positive current when the signal on the gate is low.
According to standard convention, positive current flows from the drain to the source node in NMOS devices, and flows from the source to the drain in PMOS devices. The source and drain node conventions are used only to help describe the structure and operation of embodiments of the invention and are not intended to limit the scope of the invention. It is possible to operate MOS transistors in reverse, especially if the source and drain regions are symmetrical. As such, the relative positions of the drain and source are not critical to the disclosed embodiments of the invention.
Semiconductor processes are continually evolving to meet demands for increased performance, reduced cost and reduced power consumption. Currently the mainstream technology for meeting these needs is silicon CMOS technology. CMOS is a particular form of MOS technology in which two types of transistors are used—NMOS and PMOS—hence the name Complementary MOS or CMOS. There are also NMOS and PMOS forms of MOS technology, which use exclusively NMOS and PMOS transistors respectively.
The feature size of CMOS circuits is being steadily reduced as manufacturers strive to be competitive on performance, cost and power consumption. The smaller the feature size (“geometry”) of a process, the lower the voltage at which circuits designed in the process can operate without having failures due to voltage breakdown.
Silicon CMOS processes use MOS transistors. MOS transistors have a channel between two terminals called the source and the drain. The current that flows between the source and drain can be controlled by changing the voltage on a third terminal, called the gate. For a given voltage between the source and the drain, the current that flows between the source and drain is a complex function of the voltage on the gate. This function is commonly divided into 3 regions that, taken together, give a good approximation to the behaviour of a MOS transistor:    1. If the voltage difference between gate and source, Vgs, is less than a threshold value, Vt, the drain-source current, Ids, varies exponentially with both Vgs and Vds (the drain-source voltage). This is commonly referred to as the ‘Subthreshold’ region.    2. If Vgs≧Vt, and also Vgs≧Vds, Ids varies linearly with both Vgs and Vds. (the ‘Linear’ region)    3. If Vds>Vgs>Vt, Ids, varies quadratically with Vgs, but is (almost) independent of Vds (the ‘Saturated’ region).This relationship is depicted in FIG. 1 for an NMOS transistor, and in FIG. 2 for a PMOS transistor.
In a digital circuit, the typical operating points of interest are the transistor being fully on or fully off, controlled by the gate voltage being either the minimum (Gnd) or the maximum (Vdd) voltage in the circuit. For an NMOS transistor, the fully on state corresponds to the gate being coupled to the most positive voltage in the circuit (Vdd), and the fully off state to the gate being coupled to the most negative voltage in the circuit (Gnd). From the above definitions, an on transistor is in the linear region, and an off transistor in the sub-threshold region. PMOS transistors have a complementary behaviour to NMOS transistors—they are fully on when their gates are coupled to the most negative voltage in the circuit (Gnd), and fully off when their gates are coupled to the most positive voltage in the circuit (Vdd).
Some digital circuit components, such as pass transistors, can degrade the Vdd and Gnd voltages discussed above. These degraded voltages are, however, still sufficient to generate the on and off states discussed above. Thus either a fill or degraded Vdd signal can serve as a logical high, and either a full or degraded Gnd signal can serve as a logical low signal for the digital circuit.
“Coupling” as used herein may be either a direct coupling between the two enumerated elements, or an indirect coupling through other elements between the enumerated elements. For example, the gate of the PMOS transistor discussed above may be directly coupled to Vdd, or the gate of the PMOS transistor may be indirectly coupled to Vdd through another PMOS transistor, or some other element. An example of this latter indirect coupling is shown in FIG. 10B, where the gate of transistor P2 is coupled to Vdd via transistor P1.
Turning to FIG. 3, since the subthreshold region has an exponential dependence of Ids on Vgs, the current drops dramatically as Vgs falls below the threshold level. When analysing the behaviour of digital circuits it is therefore common to regard an off transistor as carrying no current, and an on transistor as capable of carrying a high current. This is however an approximation, and in modern CMOS processes the validity of this approximation is under threat.
As CMOS technology moves to smaller and smaller geometries, the operating voltage of CMOS circuits is being steadily reduced to stay within the operating voltage limits of the smaller geometry processes. As the operating voltage reduces, so does the maximum voltage (Vdd) that can be coupled to the gate of an NMOS transistor in the circuit, and therefore the current that can be carried by a fully on transistor is reduced. FIG. 4 depicts the current flow through an NMOS transistor using a smaller geometry than the NMOS transistor current flow graph of FIG. 3. The operating speed of a CMOS circuit is typically determined by the rate at which charge can be moved on and off the parasitic capacitances in the circuit via the on transistors, so any reduction in the ability of the transistors to conduct current will lead to an increase in the time required to move this charge, and therefore to a reduction in the operating speed of the circuit. Thus as the operating voltage limits on transistors become smaller, the transistors become slower. It is possible to correct for this effect by reducing the threshold voltage of the transistors—the lower the threshold voltage, the higher the current that can be carried by a fully on transistor.
However, there is another effect to consider that restricts the CMOS process developer's freedom to reduce the threshold voltage. If the threshold voltage is reduced, the subthreshold region is correspondingly reduced. Therefore, an off transistor is not so far into the subthreshold region, and so the current through an off transistor (commonly referred to as the leakage current) will be increased. For example, comparing FIG. 4 with FIG. 5, FIG. 4 depicts an NMOS transistor having a threshold voltage Vt, and FIG. 5 depicts a second NMOS transistor having a threshold voltage Vt′, lower than Vt. Because of the exponential dependence of subthreshold current on gate voltage, a small reduction in threshold voltage can lead to a large increase in leakage current.
For process geometries of about 0.13 μm and below it is no longer possible to find a single choice of threshold voltage that is suitable for both high speed and low leakage circuit operation. Instead, such processes commonly offer the circuit designer a choice of two or more types of NMOS (and PMOS) transistors, with different threshold voltages.
The highest threshold voltage transistor will have the lowest leakage current when turned fully off, and will conduct the smallest current when turned fully on. The current conducted when the transistor is turned fully on will limit the performance of many circuits.
The lowest threshold voltage transistor will have the highest leakage current when turned fully off, and will conduct the largest current when turned fully on. The current conducted when the transistor is turned fully on will provide the highest performance circuits. However, the high leakage current (which may be thousands of times higher than the leakage current of a highest threshold transistor of the same size) will often mean that these low Vt transistors can only be used in portions of circuits that are time-critical, or where power dissipation is not a concern for some other reason.
Typically the designer's concern for the level of leakage current is not related to ensuring correct circuit operation, but is related to minimising power dissipation. For portable electronic devices this equates to maximising battery life. For example, mobile phones need to be powered for extended periods (known as standby mode, during which the phone is able to receive an incoming call), but are fully active for much shorter periods (known as talk or active mode, while making a call). When an electronic device such as a mobile phone is in standby mode, certain portions of the circuitry within the electronic device, which are active when the phone is in talk mode, are shut down. These circuits, however, still have leakage currents running through them, even though they have been de-activated. Even if the leakage current is much smaller than the normal operating current of the circuit, the leakage current depletes the battery charge over the relatively long standby time, whereas the operating current during talk time only depletes the battery charge over the relatively short talk time. As a result, the leakage current has a disproportional effect on total battery life, making leakage current an important design constraint.
Therefore, systems and methods are needed to allow low threshold transistors to be used where advantageous for circuit performance, while reducing the constraints on their use imposed by leakage current, for example in an idle or standby mode. Additionally, there is a need for systems and methods to minimise leakage current in active circuits, such as circuits in a run mode, by providing settings of unused portions of the active circuit that are chosen to minimise the leakage current.